1. Field of the Invention
The present invention relates generally to optical detectors and, more particularly, to imaging sensors.
2. Related Art
Systems utilizing high energy radiation, such as x-radiation and gamma radiation, to examine the internal structure of a solid object are well known. Such systems typically irradiate an object under examination with high energy x-radiation or gamma radiation and utilize detection apparatus to measure the intensity of the radiation that is transmitted through the object.
Conventional detection systems, particularly those used for medical applications, use a film to record an image of x-rays that are passed through a human body. Such a film typically includes a screen of fluorescent material that fluoresces to produce visible light radiation in response to incident high energy x-rays. The light radiation from the screen passes to a photosensitive film that reacts to the emitted visible light to physically record an image. Such films are used to provide a radiograph of the irradiated region of the body, the radiograph having a spatial resolution of up to 15 line pairs per millimeter.
Although x-ray film produces a radiograph having a relatively high spatial resolution, the intensity resolution is relatively low. The intensity resolution, or dynamic range, of film is typically less than 50. In addition, the film necessarily requires a substantial amount of time to develop, and the film requires a relatively high level of exposure of x-rays to produce a satisfactory radiograph. Also, the film image is not in a form that readily lends itself to computer storage or analysis.
Accordingly, detection systems have been developed for more rapidly recording the intensity of x-rays or other high energy radiation that are transmitted through a target object. Such systems typically employ a scintillation plate to covert incident x-rays to corresponding visible light radiation. A photodetector is typically used to generate an electrical signal corresponding to the intensity of the visible light produced. The electrical signal from the photodetector may be readily converted to a digital representation suitable for use with a computer and stored in a memory device or electronically displayed, for example, on a cathode ray tube.
Conventional electronic radiation detection devices have been used to produce electronic radiographic images much more quickly than can be achieved with film. Such systems also typically have a somewhat larger dynamic range than x-ray film systems. However, the radiographic images produced with such prior art electronic radiation detectors have not had the high spatial resolution that is characteristic of radiographic images produced on film. Furthermore, such conventional detectors produce significant electronic noise resulting in a dynamic range (intensity resolution) that is insufficient for most imaging tasks. Therefore, electronic imaging systems have not heretofore been suitable for producing high resolution radiographic images.
The present application is directed to different inventive aspects of a low noise, high spatial resolution, high dynamic range (high intensity resolution) image detection system. The following aspects of the present invention may be utilized in different detection systems and such detection systems may be suitable for different applications. For example, the disclosed aspects of the present invention may be utilized in medical imaging applications such as x-ray mammography systems, scientific imaging systems such as x-ray crystallography and astronomy, industrial quality-control systems, etc.
One aspect of the present invention includes a sensor array in an image sensor that minimizes damage and performance degradation due to shock, vibration and thermal stresses. In one embodiment, a sensor array for implementation in an image sensor is disclosed. The sensor array includes a mounting frame and a plurality of sensor modules removably mounted in the mounting frame. Each sensor module includes a high demagnification fiberoptic taper having an input surface and an output surface. The sensor module also includes a photodetector array optically coupled to the fiberoptic taper output surface to receive light photons transferred through the fiberoptic taper. The photodetector array is rigidly attached to the fiberoptic taper such that movement of the fiberoptic taper does not interfere with photodetector array operation. The sensor module also includes a flange constructed and arranged to individually mount the fiberoptic taper to the mounting frame, the flange flexibly attached to the fiberoptic taper and rigidly attached to the mounting frame. The fiberoptic tapers of the sensor modules are mounted in a non-contact arrangement in the mounting frame. In one embodiment, the photodetector array is a CCD photodetector array. Alternatively, the photodetector array may be a CID or CMOS photodetector array. In addition, each flange mechanically supports a fiberoptic taper such that the orientation of the fiberoptic taper may be individually adjusted.
Significantly, this aspect of the present invention provides the benefits associated with a modular design such as functional compactness and individual replacement and adjustment while minimizing the space consumed by the composite sensor array.
Another aspect of the invention includes a sensor array including a plurality of sensor modules each including a high demagnification taper and a photodetector array. In one disclosed embodiment, a sensor array for implementation in an image sensor is disclosed. The sensor array includes a mounting frame and a plurality of sensor modules mounted in the mounting frame. Each sensor module includes a high demagnification fiberoptic taper having a demagnification ratio of at least 3:1 and an input surface and an output surface. A photodetector array optically coupled to the fiberoptic taper output surface to receive light photons transferred through the fiberoptic taper is also included.
In one embodiment, the fiberoptic tapers have a demagnification ratio of greater than 2.4:1. In another embodiment, between 3.5:1 and 4.5:1.; in a further embodiment, greater than 4:1; in a still further embodiment, greater than 3.1:1. The use of fewer high demagnification fiberoptic tapers provides for fewer sensor modules resulting in an image detection system which is less complex, less costly and easier to maintain, than conventional systems.
Another aspect of the present invention includes a technique for eliminating direct physical contact between neighboring fiberoptic tapers in an array of fiberoptic tapers while simultaneously minimizing the loss of data due to misalignment of such fiberoptic tapers. In one embodiment, a sensor array for implementation in an image sensor having a composite resolution is disclosed. The sensor array includes a mounting frame and a plurality of sensor modules. The sensor modules are individually mounted in the mounting frame such that the sensor modules are secured in a fixed relative position that provides a predetermined gap between neighboring fiberoptic input surfaces that is less or equal to the resolution of sensor module. In one embodiment, the gap is the minimum of the CCD pixel size or the distance associated with an modulation transfer function (MTF) of the sensor array of approximately 5%. In another embodiment, the resolution of each sensor module is substantially equal to a resolution of the photodetector array in the sensor module. In this embodiment, the gap may be a predetermined percentage of the photodetector array resolution, such as approximately 50%. In an implementation where, for example the size of individual elements of the photodetector array is 50 microns, the gap is approximately 25 microns.
Advantageously, this arrangement enables the input surfaces of the fiberoptic tapers to be mechanically aligned with each other so as to capture the entire image with minimal or no data loss overcoming problems typically associated with a mosaic of fiberoptic tapers, particularly, variations due to tolerances in the manufacturing process, inconsistency of materials, etc. Photons will be received by the fiberoptic tapers surrounding the region of gap at which photons are impinged. As such, not all information content is lost. Thus, information incident in gap will result in inefficiencies in that fewer electrons per photon will be produced. Importantly, however, there is no loss of data. This provides for opportunities to convolve the information in the surrounding region to approximate the lost information. In addition, the input surfaces of the fiberoptic tapers can be aligned so as to create a substantially flat optical surface.
In addition, because there is a small space, perhaps with elastic spacers, between adjacent fiberoptic tapers, there is a reduced likelihood that adjacent fiberoptic tapers will impact each other causing damage due to mechanical vibrations or stresses introduced by thermal expansion or contraction. This is a problem common to conventional image sensors that abut the fiberoptic tapers against each other.
Another aspect of the invention includes a sensor array in which a plurality of modular sensor modules are arranged so as to facilitate repair and maintenance of individual sensor modules. In one embodiment, a sensor array for implementation is an image sensor having an image sensor surface is disclosed. The sensor array includes a plurality of sensor modules each sensor module includes a demagnification fiberoptic taper having a substantially rectangular input surface and a substantially rectangular output surface. A photodetector array is optically coupled to the output surface. A mounting frame is also included. The mounting frame is constructed and arranged to removably secure the sensor modules in a fixed relative arrangement of no more than two substantially parallel tiers of sensor modules. Each tier includes one or more adjacent sensor modules having a side of the fiberoptic taper input surface substantially parallel with a single mating line between the tiers of sensor modules.
Importantly, this approach relegates any variations among the tapers in a direction perpendicular to the tiers to the periphery of the image sensor surface. Similarly, variations among the tapers in the opposing direction, which in the aggregate result in different lengths of the two tiers, are also relegated to the periphery of the image sensor surface. By relegating the variations in taper dimensions to the periphery of the image sensor, the present invention eliminates distracting artifacts which may appear in the broad middle region of the resulting image. Such discontinuities, which are common in conventional systems, are found to be extremely distracting to the technician interpreting the resulting image.
An additional benefit associated with this arrangement is that all sensor modules have a side that forms the periphery of sensor array. As such, sensor modules are physically accessible and can be replaced quickly and easily without exposing neighboring sensor modules to damage.
Another aspect of the present invention includes a mammography image sensor having an array of fiberoptic tapers and a scintillation plate that significantly increases the number of photons entering the fiberoptic tapers in response to a given radiation photon. In one embodiment, an image detector having a radiation source is disclosed. The image detector includes an array of sensor modules disposed in a light-tight box having an x-ray transparent front window through which radiation is received. Each sensor module includes a fiberoptic taper having a demagnification ratio, and a photodetector array optically coupled to the fiberoptic taper. The photodetector array has a plurality of photodetector elements. The image detector also includes a scintillation plate interposed between the transparent front window and the fiberoptic tapers. The scintillation plate includes a reflective substrate and a phosphor layer deposited on the reflective substrate such that the phosphor layer is distal to said reflective substrate relative to the radiation source. Preferably, the substrate has a low x-ray absorption cross-section. In one embodiment, for example, the substrate is aluminized MYLAR(copyright).
The phosphor layer may be formed by depositing phosphor grains onto the reflective substrate. Preferably, the phosphor layer has a thickness sufficient to provide a spatial resolution that is approximately equal to that of the sensor modules. In one embodiment, the thickness of the phosphor layer is approximately the same or less than an effective pixel size of the sensor module. The effective pixel size of the sensor module is defined as the product of a demagnification ratio of the fiberoptic tapers and a pixel size of the photodetector array. In one implementation, the thickness of the phosphor layer is approximately 40 xcexcm.
In another embodiment, the scintillation plate further includes a balloon having a low x-ray absorption cross-section interposed between the x-ray transparent front window and the phosphor reflective substrate. The balloon is inflated so as to retain the phosphor layer in contact with the fiberoptic tapers.
This aspect of the present invention provides advantages that have not been provided in conventional medical imaging systems. The inclusion of a reflective surface with a phosphor layer through which the x-rays are received to directly reflect the scattered photons toward the photodetectors has not been implemented in mammography systems due to the accepted understanding that such techniques adversely affect the spatial resolution of the implementing image sensor. The absence of such techniques has not been perceived as detrimental to traditional mammography detectors, however, since such detectors generally include a large number of fiberoptic tapers, reducing the requisite amount of photons needed to be generated by the phosphor layer. In this aspect of the present invention, the spatial distribution of the light produced is normally distributed about the location at which the x-ray photon impinges on the phosphor screen.
A still further aspect of the invention includes concentric screw pairs providing for independently adjusting attached members in full six degrees of freedom using a significantly small space and with a minimal quantity of components. In one embodiment, one or more concentric screw pairs are provided for adjusting the orientation and position of a proximal and distal member relative to each other. Each proximal member has a threaded passageway and each distal member has a threaded bore aligned with the threaded passageway. Each of the concentric screw pairs includes an outer screw threadably connected to the proximal member. The outer screw has a central lumen extending axially therethrough. In addition, the outer screw extends through the proximal member passageway to seat against the distal member. An inner screw is disposed freely within the central lumen such that a distal end of the inner screw extends through the proximal member to threadably mate with the bore of the distal member. The lumen has an inner diameter and the inner screw has an outer diameter that are sized and dimmensioned to provide a predetermined amount of relative lateral translation between the proximal and distal members.
In one implementation, four pairs of concentric adjustment screws are circumferentially arranged around a central pivot region of the proximal and distal members. Selectively adjusting individual concentric screw pairs causes a translation and/or a rotation of the proximal and distal members to achieve a desired relative orientation and position. Preferably, the inner and outer screws each have a control head for manual adjustment, although other control heads may be provided. Advantageously, the associated time to adjust the individual adjustment screws and the space necessary to support such an approach in conventional systems adversely affects the cost, complexity and ease of use of the implementing image sensor.
When implemented in the above-noted image detector many advantages can be achieved. For example, when implementing four such concentric screw pairs to secure each flange to the mounting frame, the imaging surface defined by the fiberoptic taper input surfaces can be maintained substantially flat by individually adjusting the orientation and position of each sensor module. This will contribute to insuring the fiberoptic tapers are appropriately spaced to avoid loss of data and to avoid image distortions and sensor module boundaries.
A further aspect of the invention includes thermoelectric cooling modules that maintain continually a constant thermodynamic connection between a heat generating element, a thermoelectric cooler and a heat sink. In one embodiment, a cooling apparatus for cooling a movable heat generating element and for transferring heat to a stationary heat sink is disclosed. The cooling apparatus includes a thermoelectric cooling device having a cold surface and a hot surface; and two thermal coupling devices. A first thermal coupling device is constructed and arranged to thermally couple the cold surface of the thermoelectric cooling device to the heat generating element. A second thermal coupling device is constructed and arranged to thermally couple the hot surface of the thermoelectric cooling device to the heat sink. At least one thermal coupling device allows for six-degree-of freedom relative movement between the thermally coupled elements.
In one application, the heat generating element is a photodetector array in one of a plurality of sensor modules also comprising a fiberoptic taper having an input surface and an output surface to which the photodetector array is optically coupled. Preferably, the photodetector array and cooling module are located in a hermetically sealed chamber. In such an embodiment, the thermoelectric cooler maintains the temperature of the photodetector array at between approximately 0xc2x0 C. to xe2x88x9245xc2x0 C.
In one embodiment, the first thermal coupling device freely thermally couples the thermoelectric cooling device and the photodetector array. The second thermal coupling device fixedly secures the hot surface of the thermoelectric cooling device to the heat sink. This provides a constant thermal coupling between the thermoelectric cooling device and the photodetector array while allowing six degree relative movement therebetween. The first thermal coupler includes a conductive block thermally coupled to the photodetector array so as to allow for relative lateral movement therebetween. A piston and a cylinder that are sized and dimensioned to enable the piston to move freely within the cylinder while maintaining thermal coupling therebetween is also included. The piston is mechanically and thermally coupled to the conductive block and the cylinder is mechanically and thermally coupled to the cold surface of the thermoelectric cooling device. The piston is biased to cause the conductive block to be held continuously against the photodetector array, and wherein the piston is coupled to the conductive block so as to allow for rotational movement between the conductive block and the photodetector array. Preferably, the piston is coupled to the conductive block via a ball joint interface.
In another embodiment, the first thermal coupling device fixedly secures and thermally couples the cold surface of the thermoelectric cooling device and the photodetector array. The second thermal coupling device freely secures the hot surface of the thermoelectric cooling device to the heat sink so as to provide constant thermal coupling between the thermoelectric cooling device and the heat sink while allowing six degree relative movement therebetween. In one particular embodiment, the second thermal coupling device includes a thermal block thermally and rigidly coupled to the hot side of the thermoelectric cooling device. The thermal block includes a passageway through which coolant travels. Coolant supply and return lines are coupled to opposing ends of the passageway in the thermal block. Each of the supply and return line includes a pair of concentric pipes arranged so as to prevent turbulence from being induced in the coolant flow. The supply and return lines each include a flexible exterior pipe, connected to the conductive block, having a flexible region along a portion of its length to provide a predetermined flexibility between the photodetector array and the heat sink, an internal lumen with a beveled region in which an interior diameter of the internal lumen decreases to a first diameter at a neck region of the exterior pipe adjacent to the thermally conductive block. The supply and return lines also include a rigid interior pipe connected to a stationary mounting plate and securely attached to the heat sink, the interior pipe extending through the lumen of the exterior pipe past the flexible region to the beveled region.
It is well known that photodetector arrays have an inherent electronic noise due to the presence of thermal noise, and that such thermal noise is a function of the ambient temperature. This aspect of the present invention reduces significantly such noise produced by the photodetector arrays. By increasing the signal-to-noise ratio of the photodetector array to be greater than that which is typically characteristic of conventional image detectors, an implementing image sensor has greater intensity and spatial resolution than conventional image sensors.
Furthermore, this aspect of the present invention overcomes the well known problems associated with flexible tubing of inevitable leakage over time due to use of microscopically porous materials or the creation of turbulence in the coolant flow that accelerates the deterioration of the tubing. In addition, the chamber prevents condensation from accumulating on photodetector arrays which, at the above-noted temperatures, will subsequently freeze and damage the photodetector arrays.
A still further aspect of the present invention includes an exposure control system that determines automatically and in real-time when a desired radiation dose is achieved. In one embodiment, a real-time automatic exposure control system for controlling a radiation source in an image sensor is disclosed. The image sensor includes a sensor array having a plurality of fiberoptic tapers each with an input surface for receiving light photons and an output surface optically coupled to a photodetector array. A portion of the light photons traveling through the fiberoptic tapers from the input surface to the output surface is detected by the photodetector array. The exposure control system includes one or more photo detectors connected to predetermined locations of an exterior surface of each of the plurality of fiberoptic tapers to detect escaping photons. Preferably, eight photo detectors concentrically are distributed around a periphery of each of the plurality of fiberoptic tapers.
In one embodiment, the exposure control system also includes an exposure control circuit lectrically coupled to said one or more photo detectors, said exposure control circuit constructed and arranged to integrate current generated by said one or more photo detectors to determine an accumulated radiation dose at predetermined locations across an image. Preferably, the exposure control circuit is further constructed and arranged to compare a voltage resulting from said integration with a preacquired characterization of the sensor array, said characterization associating a plurality of radiation doses with responsively-generated voltage values.
In another aspect of the invention the exposure control system includes electrified regions of neighboring plates of an anti-scatter grid interposed between the radiation source and the sensor array. In one embodiment, the plates are each divided into several independently electrified regions, with each region generating a separate current to provide spatial resolution of the exposure level in the direction of said plates. In certain implementations, the exposure control system also includes an exposure control circuit electrically coupled to said electrified regions of said anti-scatter grid constructed and arranged to integrate current generated by said one or more electrified regions to determine an accumulated radiation dose at predetermined locations across an image.
Advantageously, this aspect of the present invention enables an implementing detection system to generate a efficacious radiation dose to obtain an image with a single exposure that is limited in duration due to the spatial sampling provided by multiple photo detectors or electrified regions of the anti-scatter grid. In mammography systems, for example, this insures that the exposure of the region of the breast having the greatest density will be measured and considered in determining the exposure duration. Thus, the present invention minimizes patient exposure to radiation as well as the total time for performing the mammography procedure.
A still further aspect of the present invention includes a hybrid technique for transferring digital image data with minimal wires. In one embodiment, an apparatus for transmitting image data from an image sensor to a computer is disclosed. The apparatus includes a plurality of data transmission wires each for transmitting one of a plurality of bits of the image data; a ground wire for establishing a common reference potential for each of the plurality of data transmission wires. In addition, two wires for transmitting a data available (DAV) signal as a fully differential mode signal are included. The DAV signal controls the transmission of the image data. The plurality of data transmission wires may include, for example, sixteen data transmission wires for transmitting 16 bits of image data.
This aspect of the present invention minimizes the number of wires utilized while ensuring accurate and complete data transfer. For transferring image data, this aspect of the invention significantly reduces the likelihood of misregistration since the DAV signal has a high degree of integrity due to the 2 wire differential mode. The data lines are transmitted using a minimal lines that may result errors. However such errors have a minimal adverse impact on the integrity of the image data.
a still further aspect of the present invention includes An image processor having noise correction and intensity and spatial distortion correction. In one embodiment, an image processor for processing a plurality of uncorrected image data received from an imaging system is disclosed. The image processor includes a noise compensator configured to subtract from each of the plurality of uncorrected image data a corresponding pixel value in a dark image representing noise generated by the imaging system to generate a baseline corrected image data. The image processor also includes an intensity distortion correction system configured to apply to the baseline corrected image data an intensity correction value to generate intensity corrected image data, as well as a spatial distortion correction system configured to apply to the intensity corrected image data a spatial distortion correction value to generate corrected image data representing an intensity corrected and spatially undistorted image.
a further aspect of the present invention includes a method of generating intensity correction data using a flood and dark images. In one embodiment, a method of generating intensity correction data representing the intensity distortions of an image sensor is disclosed. The method includes the steps of: exposing the image sensor to an x-ray source having a known x-ray distribution with no target present between the x-ray source and the image sensor to generate a flood image; exposing the image sensor to the x-ray source producing no x-rays with no target present between the x-ray source and the image sensor to generate a dark image; calculating an ideal flood image that would be created by the x-ray source given its known x-ray distribution field; calculating a dark subtracted flood image by subtracting the dark image from the flood image; and dividing the ideal flood image by the dark-subtracted flood image to determine intensity correction values for each pixel of the image sensor.
a still further aspect of the present invention includes a method of spatially correcting a spatially distorted image using precalculated convolution kernel data. In one embodiment a method of correcting spatial distortions in a plurality of image data received from an image sensor is disclosed. The method includes the steps of: characterizing spatial distortions of the image sensor to generate a plurality of transformation data between pixel locations in a distorted and undistorted image; selecting a pixel in the undistorted image, the pixel having a pixel location; determining a corresponding pixel location in the distorted image using the transformation data; selecting precalculated convolution function, wherein the convolution function is represented by a data structure of precalculated convolution kernel data; determining an intensity of pixels in a neighborhood of the corresponding pixel based on the convolution function; and convolving the intensities of the pixels in the neighborhood of the corresponding pixel with the convolution function to determine the intensity of the corresponding pixel. Preferably, the transformation data is run length encoded.
Various embodiments of the present invention provide certain advantages and overcome certain drawbacks of the conventional techniques. Not all embodiments of the invention share the same advantages and those that do may not share them under all circumstances. This being said, the present invention provides numerous advantages including those noted above. Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numerals indicate identical or functionally similar elements. Additionally, the left-most one or two digits of a reference numeral identifies the drawing in which the reference numeral first appears.